The present invention relates to an optical disk on and from which information is recorded and reproduced by irradiating a light beam and also to a recording/reproducing method thereof.
An optical disk memory which achieves reproducing or recording/reproducing of information by irradiation of a light beam has been put into practical use as a storage medium for audio, video and computer data files which realizes a large capacity, high-speed access, and portability. The optical disk memory is therefore expected to develop more in the future. High-density techniques for an optical disk are considered to involve shortening of the wavelength of a gas laser for cutting a master disk, shortening of the wavelength of a semiconductor laser as an operating light source, improvement of the numerical aperture of an objective lens, and thinning of an optical disk. Further, in a recordable optical disk, there are much more various approaches such as mark length recording, land-groove recording, and the like.
As a technique capable of effectively achieving high-density of an optical disk, a proposal has been made as to a super-resolution reproduction technique which utilizes medium films. The super-resolution reproduction technique has originally been proposed as a technique peculiar to a magneto-optical disk. In the super-resolution reproduction technique for a magneto-optical disk, a magnetic film (or super-resolution film) is provided in the incident side of a reproduction beam with respect to a recording layer, and both of the super-resolution film and the recording layer are exchange-coupled or magneto-statically coupled. Then, a reproduction beam is irradiated to increase the temperature of the super-resolution film thereby to change the exchange force or magneto-static force so that an optical mask or an optical aperture is formed in the super-resolution film. The super-resolution reproduction is thus realized.
Thereafter, reports were made as to attempts to provide a super-resolution film whose transmittance changes due to irradiation of a reproduction beam in the incident side of the reproduction beam with respect to a recording layer to perform super-resolution reproduction in case of a ROM disk in addition to a magneto-optical disk. Thus, it has been revealed that the super-resolution reproduction technique is applicable to all optical disks including a magneto-optical disk, CD-ROM, CD-R, WORM, phase change optical disk, and the like.
Super-resolution reproduction techniques are classified into a heat mode system and a photon mode system. Explanation will now be made as for examples of conventional super-resolution films which have already been proposed.
In the heat mode system, a phase change material is used as a super-resolution film. The super-resolution film is heated by irradiation of a reproduction beam to cause a phase change so that an optical aperture smaller than the reproduction beam spot is formed. The shape of the optical aperture follows the isotherm of the super-resolution film. However, since the size of the optical aperture easily changes due to influences from the environmental temperature, it is necessary to perform heat control strictly in compliance with the linear velocity of the optical disk. In addition, it is difficult to obtain sufficient repetition stability in the super-resolution film of the heat mode system because of thermal fatigue caused by reproduction and recording.
In the photon mode system, a photochromic material is used as a super-resolution film, and development or erasure of color based on irradiation of a reproduction beam is utilized to form an optical mask or an optical aperture. In the photochromic material, electrons are excited from a ground level to a excitated level of short lifetime by irradiation of light, and further, electrons are transmitted from the excited level to a metastable excitated level of very long lifetime in which they are captured thereby to cause a change of light absorption characteristics. Therefore, in order to perform reproduction repeatedly, electrons captured at the metastable excitated level must be deexcited to the ground level so that the optical aperture may be closed. However, since a supplemental beam is irradiated for the deexcitation, two-beam operation is required disadvantageously in view of high-speed response. Also, the photochromic material causes transmittance change through complicated processes accompanying atomic migration or change in molecular bond, and therefore, the repetition stability is limited up to about 10,000 times.
As described above, to realize super-resolution reproduction of an optical disk, it is required that the transmittance change of the super-resolution film is caused within the range of practical reproduction power, the degree of change is large, an optical aperture can be formed within a short time equivalent to about the passing time of the reproduction beam spot, and repetitive reproduction can be performed many times. However, a conventional super-resolution film cannot meet all of those requirements.
Further, super-resolution recording has hardly been proposed although super-resolution reproduction has been studied. That is, recording marks recorded at a shortened interval can be reproduced at high resolution by super-resolution reproduction, while very small recording marks conventionally cannot be formed because the size of the recording mark is determined by the spot size of the recording beam.
An object of the present invention is to provide an optical disk capable of super-resolution recording by forming a recording mark smaller than the spot size of a recording beam defined by the wavelength of a light source and the NA of a focusing lens. Another object of the present invention is to provide an optical disk capable of super-resolution recording and also capable of super-resolution reproduction by which fine recording marks formed at a fine mark pitch can be reproduced with high resolution.
An optical disk according to the present invention has a recording layer onto which light is irradiated to perform recording/reproducing of information, and comprises a super-resolution film for recording film in which an optical aperture smaller than a spot size of a recording beam is formed by irradiating the recording beam.
In the optical disk according to the present invention, it is preferable that the super-resolution film for recording and a reflective film are provided in a side opposite to a light entering side of the recording layer into which a light beam enters. Also, it is preferable that the super-resolution film for recording and the reflective film are provided in a side opposite to a light entering side of the recording layer into which a light beam enters, and a super-resolution film for reproduction is provided in the light entering side of the recording layer.
Further, it is preferable that the super-resolution film for recording and the super-resolution film for reproduction are made of semiconductor films or semiconductor particle dispersed films which have a characteristic that absorbance thereof decreases due to light excitation, and semiconductor forming the super-resolution film for recording is adjusted to have a lower light excitation probability than semiconductor forming the super-resolution film for reproduction.
In a method of recording/reproducing with respect to an optical disk having a recording layer, a super-resolution film for recording, and a super-resolution film for reproduction, according to the present invention, recording is performed by irradiating a light beam of an intensity at a recording level thereby to form a smaller optical aperture in the super-resolution film for recording than a spot size of the light beam, and by forming a recording mark having a size corresponding to the formed optical aperture, in the recording layer, and reproduction is performed by irradiating a light beam of an intensity at a reproduction level thereby to form a smaller optical aperture in the super-resolution film for reproduction than a spot size of the light beam, and by reading out a recording mark formed in the recording layer, through the formed optical aperture.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.